The 1095 bps open reading frame of ALDOB encodes a 364 amino acids protein with a calculated molecular weight of 39.3kDa. The functional Aldolase B is a homotetramer. According to the three-dimensional structures of aldolase B homotetramers, the active sites of each monomer locate at the center of the alpha/beta barrels, while the C terminus of the protein is involed in determining the isozyme-specific activity of aldolase. Four isozyme specific regions (ISR) of aldolase B were determined, the first three are expressed by exon 3 of the human aldolase gene, the fourth locates at the C-terminal region.

Expression

There are three genetically distinct and tissue-specific isozymes of fructose-biphosphate aldolase (EC-Number 4.1.2.13 ) class-I in mammals. The A isozyme(aldolase A) is expressed mainly in muscle, the B isozyme(aldolase B) in the liver, kidney, stomach and intestine, and the C isozyme (aldolase C) in the brain, heart and ovary. Aldolase B is the only expressed isoform in highly differentiated hepatocytes. The high level of gene expression results from cooperation between a liver-specific promoter and an intronic enhancer.

Localisation

Cytoplasm and perinuclear membrane of hepatocytes.

Function

All the three aldolase isozymes catalyze the reversible cleavage of fructose-1,6-(bis) phosphate (FBP) or fructose 1-phosphate (F1P) to dihydroxyacetone phosphate and either glyceraldehyde- 3-phosphate or glyceraldehyde, respectively. Aldolase B has equal activity toward substrate F1P and FBP, and is involved in the two opposite metabolic pathways, glycolysis and gluconeogenesis. Aldolase isozymes utilize covalent catalysis through a Schiff base in the active site of the enzyme, but exhibit distinct catalytic properties. The Schiff-base lysine is located in the central cavity of the barrel. The enzymatic active sites at aldolose B protein sequence are: Arg 55 and Lys146 for binding of c-1-phosphate group of the substrate; Lys 299, the Schiff base for dihydroxyacetone-p; Try 363 for enzymatic activity toward fructose 1,6- bisphosphate site; Asp33, Glu187 and Lys229 residues for catalytic function.

Recessively inherited mutations in the ALDOB gene, that caused catalytic deficiency of aldolase B, have been found in hereditary fructose intolerance (HFI). Many types of mutation in human ALDOB gene were reported, including missense mutations, nonsense mutations, deletions, insertions and mutation at the splicing regions (list in the diagram above). The mutations bring about reduced enzyme activity and affect structural stability. Mutants that retained tetrameric structure but with altered kinetic properties would reduce its catalytic activity. Mutants with homotetramers dissociated into subunits would have more severe impaired enzymatic activity. The three most common sites are: p.A150P (64%), p.A175D (16%) and p.N335K (5%).

Somatic

Human cancer result from the genetic mutation of ALDOB was not reported so far.

Implicated in

Entity

Hereditary fructose intolerance (HFI)

Disease

An autosomal recessive disease that results in the inability to metabolize fructose and related sugars. When fructose, sucrose, or sorbitol was taken from the diet, affected patients suffer from vomiting, abdominal pain, hypoglycemia. Continued ingestion of noxious sugars leads to hepatic and renal injury, which eventually leads to liver cirrhosis and growth retardation.

Prognosis

Complete exclusion of fructose, sucrose, and sorbitol from the diet results in dramatic recovery if liver and kidney damage is not irreversible.

Aldolase B is the only expressed isoenzymes of aldolase in highly differentiated hepatocytes. The mRNA of aldolase B was downexpressed in HCC patients detected by northern blot or RT-PCR, and it was also undetectable or expressed at very low levels in the hepatocellular carcinoma (HA22T, SKHep, HCC36, PLC/PLZ/5 and Hep3B) and hepatoblastoma (HepG2) cell lines.

Disease

Hepatocellular carcinoma (HCC) is an aggressive malignancy with a poor prognosis. Down-regulation of ALDOB was detected in patients of HCC and is associated with advanced disease, ETR and poor prognosis. A dramatic down-regulation of ALDOB was found in 116 of 203 HCCs (57%), while 43% of HCCs maintained the expression. The ALDOB down-regulation correlated with high-grade (grade II-IV) HCC (p<0.0001), portal vein invasion ((stage IIIB-IV) (p<1x10-6), early tumor recurrence (ETR) (p<0.001)) and a lower 5-year survival (p=0.000001).

Prognosis

In stage II HCC which had no vascular invasion, the ALDOB down-regulation was associated with ETR (p<0.05) and a lower 5-year survival (p=0.015), and ALDOB down-regulation in stage II HCC is a predictive marker of ETR and an unfavorable outcome.